Cryostat sections of synovial biopsy tissue from patients with active rheumatoid arthritis (RA), spondyloarthritis (SpA) and osteoarthritis (OA) have been investigated for interleukin-2 (IL-2) mRNA and protein production. In situ hybridization using 32P-labelled oligonucleotide probes based on IL-2 gene sequences coded by exon 1 and exon 3 of the IL-2 gene revealed IL-2 mRNA over lymphoid aggregates in RA and SpA tissue. No IL-2 mRNA was detected in OA tissue and reflected the absence of lymphoid infiltrates in these tissues. Total mRNA, as detected by a 32P oligo dT probe, was found in all tissue examined. IL-2 protein product was detected by monoclonal antibody staining in SpA but not in RA or OA tissue. This is the first report of in situ studies of lymphoid function in the microenvironment of the RA synovium. Whether these data reflect the function of normal CD3+ CD4+ CD45RO+ T-cell subsets in the diseased joint remains to be resolved.